The present invention relates to a control system for a continuously variable automatic transmission (often abbreviated to xe2x80x9cCVTxe2x80x9d), and specifically to a continuously variable automatic transmission control system capable of switching a normal mode and a linear mode depending on a throttle opening.
In recent years, continuously variable automatic transmissions such as a segmented metal drive-belt type continuously variable transmission and a toroidal CVT are often mounted on vehicles. Japanese Patent Provisional Publication No. 4-54371 (hereinafter is referred to as JP4-54371) teaches the control of a continuously variable transmission in which a desired input rotational speed is based on a throttle opening as well as vehicle speed. According to the control disclosed in JP4-54371, when increasing the throttle opening to accelerate the vehicle, the desired transmission input speed tends to increase, thus increasing the transmission, ratio. At this time, there is a slight lag time until the actual transmission ratio begins to increase. For such a slight lag time, the driving force (drive torque) remains unchanged with the rise in transmission input speed substantially corresponding to engine speed. As a result, the driver feels uncomfortable owing to the unintended racing of the engine. To avoid this, U.S. Pat. No. 4,764,155 (corresponding to Japanese Patent No. 2593432) teaches the mode selection from a normal mode to a linear mode triggered by a throttle opening greater than a predetermined threshold value. That is, when the throttle opening is less than the predetermined threshold value, the CVT controller selects the normal mode in which the desired transmission ratio is determined based on engine/vehicle operating conditions such as engine speed and load. When the throttle opening exceeds the predetermined threshold value, the CVT controller selects the linear mode in which a desired transmission ratio is fixed to a certain transmission ratio. During the linear mode, the desired transmission ratio is actually fixed to a transmission ratio retrieved from a preprogrammed change-speed map, just when the throttle opening reaches the threshold. Owing to the transmission ratio fixed by switching to the linear mode, the driving force tends to smoothly increase according to the increase in engine speed. Thus, a time period from a time when the throttle opening starts to increase to a time when the actual acceleration feeling is created is effectively shortened, thereby enhancing the response of vehicle acceleration.
However, in the transmission control as disclosed in U.S. Pat. No. 4,764,155, when the throttle opening exceeds the threshold at a predetermined vehicle speed and therefore the operating mode is switched to the linear mode, the desired transmission ratio is fixed to a transmission ratio retrieved from the preprogrammed change-speed map just when the throttle opening reaches the threshold, irrespective of the transmission ratio actually used just before the throttle opening begins to increase. Assuming that the transmission ratio actually used just before the throttle opening begins to increase is comparatively large, there is a tendency of insufficient downshifting, thus resulting in a poor acceleration performance. Conversely, assuming that the transmission ratio actually used just before the throttle opening begins to increase is comparatively small, there is a tendency of excessive downshifting, thereby causing undesirable engine-racing feel. When fixing the desired transmission ratio during the linear mode, regardless of three factors, namely the throttle opening, a time rate of change (in particular, a time rate of increase) in the throttle opening, and the vehicle speed, it is difficult to satisfy the acceleration requirement based on the driver""s wishes. During the automatic change-speed control, It is desirable to avoid undesired damage to the CVT owing to overspeed of the engine. Recently, there have been proposed continuously variable transmissions having a manual mode in which a predetermined range gear is manually selected. During shifting in the manual mode, when selecting the manual mode, or when releasing the manual mode, it is desirable for the driver to be able to recognize the selected gear at the manual mode, the shift from D range to manual mode, or the shift from manual mode to D range, without seeing an instrument panel or a selector lever.
Accordingly, it is an object of the invention to provide a control system for a continuously variable automatic transmission, which avoids the aforementioned disadvantages.
In order to accomplish the aforementioned and other objects of the present invention, a control system for a continuously variable automatic transmission for an automotive vehicle, comprises a throttle opening sensor that detects a throttle opening, and a controller that executes a change-speed control in a normal mode in which a desired transmission input speed is determined based on operating conditions of the vehicle and a desired transmission ratio is determined based on the desired transmission input speed when the throttle opening is less than a first value, and that executes the change-speed control in a linear mode in which the desired transmission ratio is fixed to a second value when the throttle opening is greater than or equal to the first value, the second value being set based on the desired transmission ratio used in the normal mode prior to switching to the linear mode.
According to another aspect of the invention, a control system for a continuously variable automatic transmission for an automotive vehicle, comprises means for detecting a throttle opening, and a controller that executes a change-speed control in a normal mode in which a desired transmission input speed is determined based on operating conditions of the vehicle and a desired transmission ratio is determined based on the desired transmission input speed when the throttle opening is less than or equal to a first predetermined throttle opening, and that executes the change-speed control in a first linear mode in which the desired transmission ratio is fixed to a predetermined first value preprogrammed to be suitable for a predetermined first acceleration requirement when the throttle opening is greater than a second predetermined throttle opening and less than or equal to a third predetermined throttle opening and a time rate of increase in the throttle opening is less than or equal to a predetermined time rate of increase, and that executes the change-speed control in a second linear mode in which the desired transmission ratio is fixed to a predetermined second value preprogrammed to be suitable for a predetermined second acceleration requirement higher than the predetermined first acceleration requirement when the throttle opening is greater than the second predetermined throttle opening and less than or equal to the third predetermined throttle opening and the time rate of increase in the throttle opening is greater than the predetermined time rate of increase, and that executes the change-speed control in a third linear mode in which the desired transmission ratio is fixed to a predetermined third value preprogrammed to be suitable for a predetermined third acceleration requirement higher than the predetermined second acceleration requirement when the throttle opening is greater than a fourth predetermined throttle opening, the fourth predetermined throttle opening being greater than the third predetermined throttle opening, the fourth predetermined throttle opening being greater than the third predetermined throttle opening being greater than the second predetermined throttle opening being greater than the first predetermined throttle opening, each of the predetermined first, second, and third values being set based on the desired transmission ratio used in the normal mode prior to switching to either of the first, second, and third linear modes.
According to a further aspect of the invention, a method for controlling a continuously variable automatic transmission for an automotive vehicle, the method comprises detecting a throttle opening, determining, based on the throttle opening and a time rate of increase in the throttle opening, whether a running mode is a normal mode, a first linear mode, a second linear mode, or a third linear mode, executing a change-speed control in the normal mode in which a desired transmission input speed is determined based on operating conditions of the vehicle and a desired transmission ratio is determined based on the desired transmission input speed when the throttle opening is less than or equal to a first predetermined throttle opening, executing the change-speed control in the first linear mode in which the desired transmission ratio is fixed to a predetermined first value preprogrammed to be suitable for a predetermined first acceleration requirement when the throttle opening is greater than a second predetermined throttle opening and less than or equal to a third predetermined throttle opening and the time rate of increase in the throttle opening is less than or equal to a predetermined time rate of increase, executing the change-speed control in the second linear mode in which the desired transmission ratio is fixed to a predetermined second value preprogrammed to be suitable for a predetermined second acceleration requirement higher than the predetermined first acceleration requirement when the throttle opening is greater than the second predetermined throttle opening and less than or equal to the third predetermined throttle opening and the time rate of increase in the throttle opening is greater than the predetermined time rate of increase, executing the change-speed control in the third linear mode in which the desired transmission ratio is fixed to a predetermined third value preprogrammed to be suitable for a predetermined third acceleration requirement higher than the predetermined second acceleration requirement when the throttle opening is greater than a fourth predetermined throttle opening, the fourth predetermined throttle opening being greater than the third predetermined throttle opening, the fourth predetermined throttle opening being greater than the third predetermined throttle opening being greater than the second predetermined throttle opening being greater than the first predetermined throttle opening, and setting each of the predetermined first, second, and third values based on the desired transmission ratio used in the normal mode prior to switching to either of the first, second, and third linear modes.
The other objects and features of this invention will become understood from the following description with reference to the accompanying drawings.